A Shadow (影, Kage) is the embodiment of ones repressed emotions.
Shadows are the embodiment of the repressed emotions one oneself. Both humans and tsugumomo possess a shadow-self. They look identical to their original counterparts, only growing monstrous, sporting sharp teeth and shadows across themselves as they act more aggressive.
If a shadow-self is able to defeat and overtake the original their personality will be overwritten by the shadows. Attempting to ignore and deny the shadow will only cause the owner to withdraw into themselves allowing for the shadow more easily. Even if the shadow is defeated they will continue exist within the original.
Shadows tend to manifest in dreams and mirrors. Shadows-selves possess all the skills and abilities of their original selves and are able to use them to their full potential as they are not suppressed by their subconscious.